package org.bouncycastle.crypto.paddings;

import java.security.SecureRandom;

import org.bouncycastle.crypto.InvalidCipherTextException;

/**
 * A padder that adds PKCS7/PKCS5 padding to a block.
 */
public class PKCS7Padding implements BlockCipherPadding {
    /**
     * Initialise the padder.
     * 
     * @param random
     *            - a SecureRandom if available.
     */
    public void init(SecureRandom random) throws IllegalArgumentException {
        // nothing to do.
    }

    /**
     * Return the name of the algorithm the padder implements.
     * 
     * @return the name of the algorithm the padder implements.
     */
    public String getPaddingName() {
        return "PKCS7";
    }

    /**
     * add the pad bytes to the passed in block, returning the number of bytes added.
     */
    public int addPadding(byte[] in, int inOff) {
        byte code = (byte) (in.length - inOff);

        while (inOff < in.length) {
            in[inOff] = code;
            inOff++;
        }

        return code;
    }

    /**
     * return the number of pad bytes present in the block.
     */
    public int padCount(byte[] in) throws InvalidCipherTextException {
        int count = in[in.length - 1] & 0xff;

        if (count > in.length || count == 0) {
            throw new InvalidCipherTextException("pad block corrupted");
        }

        for (int i = 1; i <= count; i++) {
            if (in[in.length - i] != count) {
                throw new InvalidCipherTextException("pad block corrupted");
            }
        }

        return count;
    }
}
